Optical fibers have been widely used in communication systems to achieve high data capacity that is difficult to obtain in the radio frequency (rf) and microwave spectrum. For commonly used silica fibers, the available bandwidth extends to about 60 THz from 1.25 .mu.m to 1.66 .mu.m, of which approximately 30 THz are actually useful due to water absorption lines around 1.4 .mu.m. Demand for high bandwidth is generated, at least in part, by the recent advent in information technology for applications involving graphic, audio, and video files, and by the rapid increase in information exchange via the Internet and other electronic information channels.
High data capacity in optical links requires efficient and optimized use of available bandwidth in silica fibers. One way to increase the data capacity of a system is signal multiplexing. Multiple signals can be combined into a single channel, for example, by time-division multiplexing and frequency-division multiplexing. To further increase the capacity of an existing optical fiber link, wavelength-division multiplexing is used to multiplex different channels so that all channels can be simultaneously transmitted in the same fiber on optical carriers of different wavelengths.
Frequency division multiplexing and wavelength division multiplexing require precise and reliable frequency measurements to divide the available bandwidth of a fiber link into small bands for different carriers without crosstalk. Thus, well-determined carrier frequencies are desirable. In analogy with FM-radio broadcasting where the FCC assigns a well specified transmission frequency for each station, an overcrowded optical communications network will require accurate carrier specification. Failure to satisfy this requirement will both inhibit the performance and reduce the bandwidth capabilities of a fiber optics network.
An optical frequency standard usually is a stabilized reference laser with a frequency (or wavelength) whose absolute value is known. Other laser frequencies are measured and compared against this frequency standard. A laser can be stabilized at a pre-assigned frequency relative to a frequency standard by controlling the laser via feedback based on an error signal generated by frequency comparison. Laser stabilization is a well-established technology in the art. Once a second laser is stabilized relative to the frequency standard, the second laser becomes a frequency standard itself since its frequency and phase are locked and stabilized with respect to the original frequency standard.
FIG. 1 illustrates a block diagram for a generic optical frequency metrological system 100. A laser 110 is assigned a frequency .omega..sub.L that is separated from a frequency standard .omega..sub.S by a frequency separation .alpha. (e.g., .omega..sub.L =.omega..sub.S -.DELTA.). A portion of the output of laser 110 is fed to an optical communication frequency standard comparator 120. A frequency standard generator (FSG) 122 produces a signal at .omega..sub.s. A frequency mixer 124 receives both signals .omega..sub.L and .omega..sub.S and generates an error signal 126. The laser frequency .omega..sub.L is corrected by feeding the error signal 126 back to the laser 110, thus producing an output at the desired frequency (.omega..sub.S -.DELTA.)
In principle, a single frequency standard should suffice since any other frequencies can be measured relative thereto. This requires that optical signal mixers and detection systems be sensitive to a frequency difference .DELTA. between the frequency standard and the laser frequency. To cover the entire low ultra-violet to near infrared spectrum, the typical range required for .DELTA. is about 10 THz-1000 THz. For example, the frequency separation .DELTA. is about 250 THz for stabilizing a laser in the optic communications bandwidth at 1.55 .mu.m relative to the current optical frequency standards (e.g., in the 500-800 nm range). However, many conventional state-of-art techniques are only capable of bridging frequency differences up to about 0.3 to 0.5 THz. Fundamentals of Photonics, Ch. 18 by Saleh and Teich (1991), Wiley Series in Pure and Applied Optics, ed. J. W. Goodman.
One approach to standardizing frequencies is to use a frequency chain of multiple laser frequencies distributed over the frequency range to be bridged. The frequency distances between two adjacent lasers in this frequency chain are chosen to be small enough (e.g., less than about 0.5 THz) so that they can be measured by conventional techniques. Then by stabilizing each laser with respect to adjacent ones, the first laser at one end of the chain can be stabilized with respect to the last laser at the other end of the chain, thus establishing a bridge over a large frequency distance therebetween that otherwise cannot be measured directly by conventional techniques.
A number of existing prior-art techniques use multiple stages of conventional nonlinear wave mixing in nonlinear crystals and through electrooptic modulation to generate the frequency chains for bridging large frequency gaps and to establish additional frequency standards. For example, Telle et al. disclose a nonlinear mixing scheme in Optics Letters, Vol. 15, pp. 532, 1990; Lee and Wong describe another nonlinear mixing method in Optics Letters, Vol. 17, pp. 13, 1992.
Van Baak and Hollberg have published a number of frequency standards using conventional nonlinear mixing techniques in "Proposed sum-and-difference method for optical-frequency measurement in the near infrared", Optics Letters, Vol. 19, No. 19, pp. 1586-1588, 1994, the entirety of which is incorporated herewith by reference.
However, the spectral coverage of the frequency standards using conventional techniques is limited. For example, the established frequency standards published by Van Baak and Hollberg are mainly from 500 nm to about 800 nm, and not in the range from 1.25 .mu.m to 1.66 .mu.m, a region that is critical to fiber optic communications.
Lack of frequency standards in the fiber optic wavelength range from 1.25 .mu.m to 1.66 .mu.m has been well recognized by the industry. This lack of standards can be a limiting factor in the future and can lead to under-utilization of the available bandwidth from optical fibers. The status of optical frequency standards for optical communications and other related industrial sectors was reviewed by Knight in "Laser Frequency Standards in the Near Infrared, Coinciding with the Optical Fiber Transmission Bands", Laser Physics, Vol. 4, No. 2, pp. 345-348, 1994, and by Pollitt, "Standards to Support Light Communications", IEEE Transactions on Instrumentation and Measurement, Vol. 44, No. 2, pp.454-45, 1995. The disclosure of these two articles is incorporated herein by reference.
Conventional nonlinear mixing techniques are also limited in demodulation frequency range (e.g., less than 0.5 THz). This reduces system stability and accuracy of measurements. In addition, the limited number of existing frequency standards requires use of multiple stages of conventional nonlinear wave mixing to generate new frequency standards. This often leads to very complex cascaded nonlinear wave mixing and system performance becomes problematic due to low efficiency of many nonlinear processes and limited power available from lasers.
It has been concluded that frequency standards in 1.3 .mu.m-1.5 .mu.m range are needed for technical advances, commercial purposes, and regulatory requirements. Specifically, two frequency standards in each of the 1.3 .mu.m and 1.5 .mu.m bands (separated by unusable water absorption lines near 1.4 .mu.m) are in immediate need with an accuracy at about 1 part in 10.sup.-9.
In addition, metrology standard proliferation would be aided by additional frequency standards in a spectral range that has been covered by current frequency standards in order to increase implementation flexibility/optimization and resource utilization.
In recognition of the above, the inventors have discovered that quantum interference in multi-photon excitation can be used as a basis for a novel frequency metrological system. In accordance with the present invention, quantum interference in multi-photon excitation by multiple radiation fields at different frequencies is used to lock a laser relative to a reference frequency based on one or more known frequency standards, thereby producing a new frequency standard.
One aspect of the invention is a frequency standard generator based on quantum interference in multi-photon process. A preferred embodiment includes an absorbing medium preferably with three pre-determined excitation levels to facilitate a desired two-photon excitation process, at least one known frequency standard, and a frequency-shifting-and-mixing device. A target laser, which is to be established as a new frequency standard, operates in combination with the known frequency standard and a device for frequency mixing and/or frequency shifting to produce three excitation frequencies each corresponding to three different transitions in pre-determined excitation levels. A signal indicative of quantum interference of two different excitation paths affecting a common energy level is preferably used to control the target laser, thereby maintaining the laser frequency at a predetermined spacing relative to the known frequency standard.
Another aspect of the invention is optical demodulation of a carrier signal with a demodulation frequency range several magnitudes larger than what is obtainable in many conventional systems. For example, the two-photon transition sequence 6S.sub.1/2 .fwdarw.6P.sub.3/2 .fwdarw.6D.sub.5/2 in cesium can be used with the preferred embodiment to generate demodulation at 12.5 THz. Other two-photon transitions in cesium or atomic and molecular species can be used in accordance with the present invention to achieve optical demodulation in a range from 10's THz to 100's THz. For example, considering two-photon processes in alkalis elements (e.g., Li, Na, K, Rb and Cs), the inventors discovered demodulation possibilities that extend to about 240 THz.
Still another aspect of the invention is a method for selecting a number of atomic and molecular species as the absorbing medium, thereby making it easier to achieve a wide range of new frequency standards based on a limited number of known frequency standards. In particular, a plurality of new frequency standards can be generated to cover a spectral range from 0.2 .mu.m to 2.0 .mu.m including the 1.25 .mu.m-1.66 .mu.m spectral range for fiber optics. A computerized optimization selection process can be implemented to choose a suitable absorbing medium with desired energy levels.